1. Field of the Invention
The present invention relates to solid-state gas sensors for the detection of a chemical species. More particularly, the present invention relates to solid-state electrochemical gas sensors for detecting contaminant gas species and methods for fabricating electrochemical gas sensors.
2. Description of Related Art
Sensors for the detection of chemical species are utilized in myriad applications. For example, the detection of noxious gaseous species such as carbon monoxide (CO), hydrogen sulfide (H2S), volatile organic carbons (VOCs) or nitrogen oxides (NOx) is desirable so that a signal can be generated indicating the presence of such species. Appropriate steps can then be taken to mitigate their effect or to remove persons from the presence of the gaseous species.
Electrochemical sensors for the detection of gaseous species typically utilize large electrodes and liquid electrolytes. Acid electrolytes such as sulfuric acid are the most common liquid electrolytes, although other inorganic and organic liquids have also been utilized. However, sensors based on liquid electrolytes are known to leak under certain operating conditions and are affected by long exposures to very high or very low humidity levels. Sensors utilizing liquid electrolytes must be adequately sealed to prevent leakage of the liquid electrolyte, yet also permit the gaseous species to reach the working electrode/electrolyte interface. This requires a costly and complex sensor design and the effective lifetime of the sensors is still typically limited.
As used herein, an electrochemical sensor is a sensor in which the chemical constituent of interest (i.e., the analyte) is contacted with a catalytic electrode so that the chemical constituent is either oxidized or reduced with the exchange of electrons. The flow of electrical current due to the oxidation and reduction of the chemical constituent is used as a measure of the concentration of the constituent being detected.
One type of electrochemical gas sensor, which is sometimes referred to as an amperometric gas sensor, typically includes three electrodes in contact with an electrolyte. A working electrode is typically fabricated from platinum (Pt) or gold (Au). The gaseous species diffuses to the point where the working electrode and the electrolyte are in contact, where an electrochemical oxidation or reduction reaction occurs resulting in the capture or release of electrons. A counter electrode is used to maintain a charge balance in the sensor and the charge difference (i.e., the current flow) between the working electrode and the counter electrode generates an output signal in the form of an electric current that is proportional to the concentration of the gaseous species. In addition, a reference electrode can be used to control the operation of the sensor by maintaining a selected potential relative to the working electrode. Two electrode configurations are also utilized, where a single electrode functions as both a counter electrode and a reference electrode.
Solid electrolytes have also been utilized for electrochemical sensors. For example, ceramic electrolytes such as yttria stabilized zirconia (YSZ) are known, but require an operating temperature in excess of about 300° C., thereby requiring an on-board heater and substantial power input which render the devices unsuitable for many applications. An example of this type of sensor is disclosed in U.S. Pat. No. 6,613,207 by De La Prieta et al.
Another approach for solid electrolytes is the use of a proton conductive material such as a sulfonated tetrafluoroethylene copolymer, for example NAFION™ (E.I. duPont deNemours, Wilmington, Del.). An example of this type of sensor is disclosed in U.S. Pat. No. 5,215,643 by Kusanagi et al. However, these electrolyte materials require a constant humidity environment to retain adequate conductivity and therefore are not well suited for use in low or very high humidity environments.
U.S. Pat. No. 4,925,544 by Goldring discloses a sensor that includes an electrolyte separated from the analyte by a selectively permeable membrane, where the electrolyte is an electrically conductive solid including a homogeneous dispersion of a polymeric matrix phase and an electrically conductive salt. The polymer matrix is substantially free of water to avoid variability in the sensor due to evaporation of water during use. The polymeric matrix phase can be plasticized, the plasticizer forming a continuous phase in which the conductive salt is dissolved. The sensor is particularly useful for the measurement of blood gases.
U.S. Pat. No. 6,202,471 by Yadav et al. discloses a multilaminate sensor that includes multiple sensing layers and electrodes in a laminated stack. The sensing layers are fabricated from a material having a material property that changes when exposed to the chemical species of interest, and the material property change is measured by the electrodes.
There remains a need for an electrochemical gas sensor that is capable of operating over a range of moderate temperatures. There is also a need for an electrochemical gas sensor that is capable of operating over a wide range of humidity conditions such that the sensor can adequately function in arid environments as well as in humid environments. There is also a need for an electrochemical gas sensor having a small size and that does not require heat input or other large power input for operation.